In recent years, the number of electric parts and that of electric apparatuses mounted on a vehicle have rapidly increased owing to the fact that the vehicle has come to have a high function and a high performance. Therefore wiring inside the vehicle has become complicated and large-scaled.
To suppress an increase of the number of wires inside the vehicle, a multiple communication system is increasingly adopted. In the multiple communication system, electronic control units (hereinafter referred to as ECU) for controlling electric parts mounted on the vehicle are divided into a group for the vehicle body such as a seat, a door, and the like and another group for the power train such as an engine, a throttle, and the like. The ECUs of each group are connected to one another via multiple communication lines to construct a network so that the number of wires is decreased. Further a relay control unit (gateway unit) for relaying signals transmitted and received between the ECUs belonging to different networks is interposed between the networks.
When any one of a plurality of the ECUs connected to the multiple communication line fails to generate communication abnormality, it may cause interference with the control of the operation of a vehicle and electric parts. Therefore it is preferable to rapidly start an operation of coping with the communication abnormality by specifying the ECU which has generated the communication abnormality in an early stage.
As disclosed in Japanese Patent Application Laid-Open No. 2003-143164 (patent document 1), the ECU having the communication abnormality determining part which determines whether the communication abnormality has occurred is proposed. Each ECU transmits signals cyclically to the multiple communication lines. Thus each ECU measures an elapsed period of time from the time when the ECU receives the signal from other ECU until the ECU receives a signal subsequently transmitted from the other ECU. When the elapsed period of time is larger than the predetermined specified time, the communication abnormality determining part determines that a communication delay or communication stop has occurred and that the communication abnormality has occurred.
Patent document 1: Japanese Patent Application Laid-Open No. 2003-143164